Sustained activation of the mitogen-activated protein (MAP) kinase cascade may be required for differentiation of PC12 cells. Comparison of the effects of nerve growth factor and epidermal growth factor

EGF-induced sustained tyrosine phosphorylation and decreased rate of down-regulation of EGF receptor in PC12h-R cells which show neuronal differentiation in response to EGF

PC12h-R cell, a subclone of PC12 cells, exhibited a neuron-like phenotype, including neurite outgrowth and increased acetylcholinesterase activity, in response to epidermal growth factor (EGF) as well as nerve growth factor (NGF). We examined the mechanism by which EGF induced the neuronal differentiation in PC12h-R cells. The EGF-induced neuronal differentiation of PC12h-R cells was not blocked by K252a, whereas that induced by NGF was. EGF induced sustained tyrosine phosphorylation of the EGF receptor in PC12h-R cells, but not in the parent PC12h cells, which do not show neuronal differentiation in response to EGF. In addition, the rate of EGF-induced down-regulation of the EGF receptor in PC12h-R cells was decreased compared with that in PC12h cells. Furthermore, we found that the duration of EGF-induced tyrosine phosphorylation of the EGF receptor in PC12h-R cells was similar to that of NGF-induced tyrosine phosphorylation of p140trkA in PC12h cells. The EGF-induced phosphorylation of the EGF receptor in PC12h cells was less sustained than that of p140trkA by NGF in PC12h cells. These findings suggested that the EGF-induced neuronal differentiation of PC12h-R cells is due to the sustained activation of the EGF receptor, resulting from the decreased down-regulation of the EGF receptor and that the duration of the receptor tyrosine kinase activity determines the cellular responses of PC12 cells. We concluded that sustained activation of the receptor tyrosine kinase induces neuronal differentiation, although transient activation promotes proliferation of PC12 cells.

The pool of map kinase associated with microtubules is small but constitutively active

Mitogen-activated protein kinase (MAPK) is activated by many kinds of stimuli and plays an important role in integrating signal transduction cascades. MAPK is present abundantly in brain, where we have studied its association with microtubules. Immunofluorescence of primary hippocampal neurons revealed that MAPK staining co-localized with microtubules and biochemical analyses showed that MAPK co-purified with microtubules. Approximately 4% of MAPK in cytosolic extracts was associated with microtubules, where it was associated with both tubulin and microtubule-associated proteins (MAPs) fractions. Further fractionation of MAPs suggested that a portion of MAPK is associated with MAP2. An association with MAP2 was also demonstrated by co-immunoprecipitation and in vitro binding experiments. A similar association was shown for the juvenile MAP2 isoform, MAP2C. The pool of MAPK associated with microtubules had a higher activity relative to the nonassociated pool in both brain and proliferating PC12 cells. Although MAPK was activated by nerve growth factor in PC12 cells, the activity of microtubule-associated MAPK did not further increase. These results raise the possibility that microtubule-associated MAPK operates through constitutive phosphorylation activity to regulate microtubule function in neurons.

Induction of neurite outgrowth by interleukin-6 is accompanied by activation of Stat3 signaling pathway in a variant PC12 cell (E2) line

PC12-E2 cells, a stable variant subcloned from native cell populations, produce neurites in a rapid, transcription-independent manner upon exposure to nerve growth factor (NGF) or basic fibroblast growth factor (bFGF). They also give a similar morphological response to interleukin-6 (IL-6), which is, however, transcription-dependent and with a slower onset, a phenomenon basically not observed in native PC12 cells. The response profile of PC12-E2 cells to NGF and bFGF is similar to that observed for native PC12 cells pre-exposed (primed) to NGF, and such cells also respond to IL-6 in a fashion indistinguishable from PC12-E2 cells. Mechanistically, NGF and bFGF induce a sustained phosphorylation and activation of ERK1 and ERK2 in both cells, while IL-6 produces only a transient and weak tyrosine phosphorylation. However, it does stimulate a prolonged and biphasic tyrosine phosphorylation and nuclear translocation of Stat3 (signal transducers and activators of transcription 3; at least 24 h) and, to a lesser extent, Stat1. Gel shift and supershift analyses confirm that IL-6 predominantly activates Stat3 (and some Stat1) and stimulates sis-inducible element binding activity. Other members of the same cytokine subfamily, including ciliary neurotrophic factor and leukemia inhibitory factor, also cause a transient initial phase of tyrosine phosphorylation and activation of Stat1 and Stat3 (up to 1 h) but fail to stimulate a second phase of response and do not produce significant neurites. These results suggest that sustained signaling of either STAT or ERK pathways in PC12-E2 cells leads to induction of neuronal differentiation. However, only the latter is effective in native PC12 cells as the activation of Stat3 and Stat1 in native PC12 cells by IL-6 fails to induce neuronal differentiation. Thus, the response of PC12-E2 cells to IL-6 suggests the constitutive expression of a required factor(s) for differentiation, that is induced in native PC12 cells by NGF or bFGF (possibly by ERK activation), but not by IL-6 via Janus kinase/STAT activation. This factor(s), which has a sufficient half-life to allow primed cells to remain responsive to IL-6 for several days, is necessary but not sufficient for differentiation (as measured by neurite proliferation) to occur.

Identification of tyrosine residues in the intracellular domain of the growth hormone receptor required for transcriptional signaling and Stat5 activation

The binding of growth hormone (GH) to its receptor results in its dimerization followed by activation of Jak2 kinase and tyrosine phosphorylation of the GH receptor itself, as well as Jak2 and the transcription factors Stat1, -3, and -5. In order to study the role of GH receptor tyrosine phosphorylation in intracellular signaling, we constructed GH receptors in which combinations of tyrosines were mutated to phenylalanines. We identified three tyrosine residues at positions 534, 566, and 627 that were required for activation of GH-stimulated transcription of the serine protease inhibitor (Spi) 2.1 promoter. Any of these three tyrosines is able to independently mediate GH-induced transcription, indicating redundancy in this part of the GH receptor. Tyrosine phosphorylation was not required for GH stimulation of mitogen-activated protein (MAP) kinase activity or for GH-stimulated Ca2+ channel activation since these pathways were normal in cells expressing a GH receptor in which all eight intracellular tyrosines were mutated to phenylalanines. Activation of Stat5 by GH was, however, abolished in cells expressing the GH receptor lacking intracellular tyrosines. This study demonstrates that specific tyrosines in the GH receptor are required for transcriptional signaling possibly by their role in the activation of transcription factor Stat5.

Differential regulation of Raf isozymes by growth versus differentiation inducing factors in PC12 pheochromocytoma cells

PC12 pheochromocytoma cells possess four known MEK activators: A-, B-, c-Raf-1 and MEKK. In order to examine whether differentiation factors or growth factors have a Raf isozyme preference for activation of the mitogenic cytoplasmic Raf-MEK-MAPK protein kinase cascade, the activation kinetics of these enzymes in response to epidermal growth factor (EGF) and nerve growth factor (NGF) were compared. An initial activation of all three Raf kinases was noticed, but only A- and B-Raf showed sustained activation by NGF, which was not seen after EGF treatment. Furthermore, expression of oncogenic versions of all three Raf kinases as well, as a potentially Raf-independent MEK activator, v-Mos, leads to activation of MAPK and to differentiation of PC12 cells. These data suggest a differential regulation of Raf kinases and that probably no alternative Raf substrates are involved in differentiation processes of PC12 cells.

Lactosylceramide stimulates Ras-GTP loading, kinases (MEK, Raf), p44 mitogen-activated protein kinase, and c-fos expression in human aortic smooth muscle cells

Previously, our laboratory has shown that lactosylceramide (LacCer) can serve as a mitogenic agent in the proliferation of aortic smooth muscle cells "a hallmark in the pathogenesis of atherosclerosis" (Chatterjee, S. (1991) Biochem. Biophys. Res. Commun. 181, 554-561). Here we report a novel aspect of LacCer-mediated signal transduction. We demonstrate that LacCer (10 microM) can stimulate the phosphorylation of mitogen-activated protein (MAP) kinase p44MAPK to phosphorylated p44MAPK in aortic smooth muscle cells from rabbit or human origin. Western immunoblot assays and direct measurement of activity in immunoprecipitated MAP kinase revealed that within 5 min of incubation of cells with LacCer there was a 3.5-fold increase in the activity of p44MAPK. This continued up to 10 min of incubation; thereafter, the MAP kinase activity decreased in these cells. Phosphoamino acid analysis revealed that the tyrosine and threonine moieties of p44MAPK was phosphorylated by LacCer. Incubation of cells with ceramide and glucosylceramide did not significantly stimulate p44MAPK activity. Preincubation with tyrphostin (20 microM; a potent and specific inhibitor of tyrosine kinase) markedly inhibited the LacCer mediated stimulation in p44MAPK activity. Next we investigated the upstream and downstream parameters in MAP kinase signaling pathways. We found that lactosylceramide stimulated (7-fold) the loading of GTP on Ras. Concomitantly, LacCer stimulated the phosphorylation of MAP kinase kinases (MEK) and Raf within 2.5 min. Lactosylceramide specifically induced c-fos mRNA expression (3-fold) in these cells as compared to control. In summary, one of the biochemical mechanisms in LacCer mediated induction in the proliferation of aortic smooth muscle cells may involve Ras-GTP loading, activation of the kinase cascade (MEK, Raf, p44MAPK), and c-fos expression.

p21ras signaling is necessary but not sufficient to mediate neurotrophin induction of calcium channels in PC12 cells

Nerve growth factor and basic fibroblast growth factor bind to and activate receptor tyrosine kinases, causing sequential signaling via the p21ras/extracellular signal-regulated kinase pathway. The necessity and sufficiency of this signaling pathway in transducing neuronal differentiation have been tested in the PC12 cell model. Although necessary for morphological changes, the sufficiency of p21ras-mediated signaling in these events has come into question. We report that growth factor induction of voltage-gated calcium channels, a hallmark of physiological differentiation, also requires p21ras-mediated signaling, but cannot be driven by p21ras activation alone. Thus, constitutive expression of the dominant negative N17ras mutant blocks growth factor-induced increases in Omega-conotoxin GVIA-sensitive, nimodipine-sensitive, and Omega-conotoxin GVIA/nimodipine-resistant calcium currents, but it does not block sodium current induction. However, manipulations that produce sustained activation of the p21ras signaling pathway and the neurite extension characteristic of morphological differentiation fail to increase calcium channel current densities. These results indicate the existence of distinct signaling requirements for morphological and physiological differentiation and further emphasize the importance of p21ras-independent signaling pathways in growth factor-mediated neuronal development.

A novel cytoplasmic dual specificity protein tyrosine phosphatase implicated in muscle and neuronal differentiation

Dual specificity protein tyrosine phosphatases (dsPTPs) are a subfamily of protein tyrosine phosphatases implicated in the regulation of mitogen-activated protein kinase (MAPK). In addition to hydrolyzing phosphotyrosine, dsPTPs can hydrolyze phosphoserine/threonine-containing substrates and have been shown to dephosphorylate activated MAPK. We have identified a novel dsPTP, rVH6, from rat hippocampus. rVH6 contains the conserved dsPTP active site sequence, VXVHCX2GX2RSX5AY(L/I)M, and exhibits phosphatase activity against activated MAPK. In PC12 cells, rVH6 mRNA is induced during nerve growth factor-mediated differentiation but not during insulin or epidermal growth factor mitogenic stimulation. In MM14 muscle cells, rVH6 mRNA is highly expressed in proliferating cells and declines rapidly during differentiation. rVH6 expression correlates with the inability of fibroblast growth factor to stimulate MAPK activity in proliferating but not in differentiating MM14 cells. rVH6 protein localizes to the cytoplasm and is the first dsPTP to be localized outside the nucleus. This novel subcellular localization may expose rVH6 to potential substrates that differ from nuclear dsPTPs substrates.

GTPase-deficient G alpha 16 and G alpha q induce PC12 cell differentiation and persistent activation of cJun NH2-terminal kinases

Persistent stimulation of specific protein kinase pathways has been proposed as a key feature of receptor tyrosine kinases and intracellular oncoproteins that signal neuronal differentiation of rat pheochromocytoma (PC12) cells. Among the protein serine/threonine kinases identified to date, the p42/44 mitogen-activated protein (MAP) kinases have been highlighted for their potential role in signalling PC12 cell differentiation. We report here that retrovirus-mediated expression of GTPase-deficient, constitutively active forms of the heterotrimeric Gq family members, G alpha qQ209L and G alpha 16Q212L, in PC12 cells induces neuronal differentiation as indicated by neurite outgrowth and the increased expression of voltage-dependent sodium channels. Differentiation was not observed after cellular expression of GTPase-deficient forms of alpha i2 or alpha 0, indicating selectivity for the Gq family of G proteins. As predicted, overexpression of alpha qQ209L and alpha 16Q212L constitutively elevated basal phospholipase C activity approximately 10-fold in PC12 cells. Significantly, little or no p42/44 MAP kinase activity was detected in PC12 cells differentiated with alpha 16Q212L or alpha qQ209L, although these proteins were strongly activated following expression of constitutively active cRaf-1. Rather, a persistent threefold activation of the cJun NH2-terminal kinases (JNKs) was observed in PC12 cells expressing alpha qQ209L and alpha 16Q212L. This level of JNK activation was similar to that achieved with nerve growth factor, a strong inducer of PC12 cell differentiation. Supportive of a role for JNK activation in PC12 cell differentiation, retrovirus-mediated overexpression of cJun, a JNK target, in PC12 cells induced neurite outgrowth. The results define a p42/44 MAP kinase-independent mechanism for differentiation of PC12 cells and suggest that persistent activation of the JNK members of the proline-directed protein kinase family by GTPase-deficient G alpha q and G alpha 16 subunits is sufficient to induce differentiation of PC12 cells.

PC12h-R cell, a subclone of PC12 cells, shows EGF-induced neuronal differentiation and sustained signaling

Unlike nerve growth factor (NGF), epidermal growth factor (EGF) does not induce neuronal differentiation but promotes proliferation of the rat pheochromocytoma PC12 cells. We found that PC12h-R, a subclone of PC12 cells, differentiated into neuron-like cells in response to EGF as well as to NGF. PC12h-R cells treated with EGF extended neurites, attenuated cell proliferation, and increased the levels of tyrosine hydroxylase protein synthesis and of acetylcholinesterase activity as those treated with NGF. The EGF-induced differentiation of PC12h-R cells was not mediated by the indirect activation of p140trkA by EGF. In addition, EGF induced the sustained tyrosine phosphorylation of the EGF receptor, mitogen-activated protein (MAP) kinases, and 46 and 52 kDa proteins, and the prolonged activation of MAP kinases in PC12h-R cells compared with the parent PC12h, which does not show EGF-induced differentiation. The response of PC12h-R cells to EGF was not simply due to an increase in the level of EGF receptor protein. These results indicated that the duration of EGF-induced signaling might determine the cellular response of PC12 cells between cell proliferation and neuronal differentiation.

Regulation of cytokine-inducible nitric oxide synthase in cardiac myocytes and microvascular endothelial cells. Role of extracellular signal-regulated kinases 1 and 2 (ERK1/ERK2) and STAT1 alpha

Adult rat ventricular myocytes and cardiac microvascular endothelial cells (CMEC) both express an inducible nitric oxide synthase (iNOS or NOS2) following exposure to soluble inflammatory mediators. However, NOS2 gene expression is regulated differently in response to specific cytokines in each cell type. Interleukin-1 beta (IL-1 beta) induces NOS2 in both, whereas interferon gamma (IFN gamma) induces NOS2 expression in myocytes but not in CMEC. Therefore, we examined the specific signal transduction pathways that could regulate NOS2 mRNA levels, including activation of 44- and 42-kDa mitogenactivated protein kinases (MAPKs; ERK1/ERK2) and STAT1 alpha, a transcriptional regulatory protein linked to cell membrane receptors. Although IL-1 beta treatment increased ERK1/ERK2 activities in both cell types, IFN gamma activated these MAPKs only in myocytes. STAT1 alpha phosphorylation, consistent with IFN gamma-induced signaling, was readily apparent in both cell types, and binding of activated STAT1 alpha from cytoplasmic or nuclear fractions from IFN gamma-treated adult myocytes to a sis-inducible element could be demonstrated by gel-shift assay. The farnesyl transferase inhibitor BZA-5B blocked activation of ERK1/ERK2 and induction of NOS2 by IFN gamma and IL-1 beta in myocytes. IL-1 beta and IFN gamma-induced NOS2 gene expression in myocytes was also down-regulated by both protein kinase C (PKC) desensitization and by the PKC inhibitor bisindolylmaleimide, implicating PKC-linked activation of Ras or Raf in the induction of NOS2 by IL-1 beta and IFN gamma in cardiac muscle cells. In CMEC, the MAPK kinase inhibitor PD 98059 blocked activation of ERK1/ERK2 and down-regulated IL-1 beta-mediated NOS2 induction, whereas activation of ERK2 in the absence of cytokines by okadaic acid, an inhibitor of phosphoserine protein phosphatases, also induced NOS2 mRNA. These data demonstrate that ERK1/ERK2 activation appears to be necessary for the induction of NOS2 by IL-1 beta and IFN gamma in cardiac myocytes and CMEC. In the absence of ERK1/ERK2 activation by IFN gamma in CMEC, phosphorylation of STAT1 alpha is not sufficient for NOS2 gene expression. These overlapping yet distinct cellular responses to specific cytokines may serve to target NOS2 gene expression to specific cells or regions within the heart and also provide for rapid escalation of NO production if required for host defense.

Plasmalopsychosine of human brain mimics the effect of nerve growth factor by activating its receptor kinase and mitogen-activated protein kinase in PC12 cells. Induction of neurite outgrowth and prevention of apoptosis

Plasmalopsychosine, a characteristic fatty aldehyde conjugate of beta-galactosylsphingosine (psychosine) found in brain white matter, enhances p140trk (Trk A) phosphorylation and mitogen-activated protein kinase (MAPK) activity and as a consequence induces neurite outgrowth in PC12 cells. The effect of plasmalopsychosine on neurite outgrowth and its prolonged activation of MAPK was similar to that of nerve growth factor (NGF), and the effect was specific to neuronal cells. Plasmalopsychosine was not capable of competing with cold chase-stable, high affinity binding of NGF to Trk A, indicating that plasmalopsychosine and NGF differ in terms of Trk A-activating mechanism. Tyrosine kinase inhibitors K-252a and staurosporine, known to inhibit the neurotrophic effect of NGF, also inhibited these effects of plasmalopsychosine, suggesting that plasmalopsychosine and NGF share a common signaling cascade. Plasmalopsychosine prevents apoptosis of PC12 cells caused by serum deprivation, indicating that it has "neurotrophic factor-like" activity. Taken together, these findings indicate that plasmalopsychosine may play an important role in development and maintenance of the vertebrate nervous system.

Transforming potentials of epidermal growth factor and nerve growth factor receptors inversely correlate with their phospholipase C gamma affinity and signal activation

The exchange of nerve growth factor receptor/Trk and epidermal growth factor receptor (EGFR) phospholipase C gamma (PLC gamma) binding sites resulted in the transfer of their distinct affinities for this Src homology 2 domain-containing protein. Relative to wild-type EGFR, the PLC gamma affinity increase of the EGFR switch mutant EGFR.X enhanced its inositol trisphosphate (IP3) and calcium signals and resulted in a more sustained mitogen-activated protein (MAP) kinase activation and accelerated receptor dephosphorylation. In parallel, EGFR.X exhibited a significantly decreased mitogenic and transforming potential in NIH 3T3 cells. Conversely, the transfer of the EGFR PLC gamma binding site into the Trk cytoplasmic domain context impaired the IP3/calcium signal and attenuated the MAP kinase activation and receptor dephosphorylation, but resulted in an enhancement of the ETR.X exchange mutant mitogenic and oncogenic capacity. Our findings establish the significance of PLC gamma affinity for signal definition, the role of this receptor tyrosine kinase substrate as a negative feedback regulator and the importance of this regulatory function for mitogenesis and its disturbance in oncogenic aberrations.

Differential modulation of mitogen-activated protein (MAP) kinase/extracellular signal-related kinase kinase and MAP kinase activities by a mutant epidermal growth factor receptor

A paradigm has been established whereby mutant tyrosine kinase receptors such as the v-erbB and v-fms gene products function as oncoproteins in the absence of ligand. A spontaneously occurring deletional mutant of the human epidermal growth factor receptor (EGFR-vIII) has been isolated from astrocytic neoplasms and transforms NIH3T3 cells in the absence of ligand. The EGFRvIII is constitutively complexed with the majority of cellular GRB2, suggesting a link to the Ras-Mitogen-activated protein (MAP) kinase pathway (D. Moscatello, R. B. Montgomery, P. Sundareshan, H. McDanel, M. Y. Wong, and A. J. Wong, submitted for publication). In this report, we document that expression of EGFRvIII in fibroblasts is associated with downstream activation of mitogen-activated protein (MAP) kinase/extracellular signal-regulated kinase (MEK) and modest activation of p42 and p44 MAP kinases. The presence of EGFRvIII suppresses activation of p42 and p44 MAP kinases by phorbol 12-myristate 13-acetate (PMA) and serum; however, MEK activation by PMA is not suppressed by EGFRvIII. Basal and PMA-stimulated MAP kinase activity in EGFRvIII-transfected cells is augmented by the tyrosine phosphatase inhibitor sodium vanadate. EGFR-vIII is capable of transducing downstream signals through MAP kinase as evidenced by activation of cytoplasmic phospholipase A2 at levels similar to that induced by intact EGFR. Our results suggest that EGFR-vIII constitutively activates downstream signal transduction through MAP kinase, and this chronic stimulation of the MAP kinase pathway may represent one means by which mutant EGFR transduces an oncogenic signal.

Activation and nuclear translocation of mitogen-activated protein kinases by polyomavirus middle-T or serum depend on phosphatidylinositol 3-kinase

Several cellular signal transduction pathways activated by middle-T in polyomavirus-transformed cells are required for viral oncogenicity. Here we focus on the role of phosphatidylinositol 3-kinase (PI 3-kinase) and Ras and address the question how these signaling molecules cooperate during cell cycle activation. Ras activation is mediated through association with SHC.GRB2.SOS and leads to increased activity of several members of the mitogen-activated protein (MAP) kinase family, while activation of PI 3-kinase results in the generation of D3-phosphorylated phosphatidylinositides whose downstream targets remain elusive. PI 3-kinase activation might also ensue as a direct consequence of Ras activation. Oncogenicity of middle-T requires stimulation of both Ras- and PI 3-kinase-dependent pathways. Mutants of middle-T incapable to bind either SHC.GRB2.SOS or PI 3-kinase are not oncogenic. Sustained activation and nuclear localization of one of the MAP kinases, ERK1, was observed in wild type but not in mutant middle-T-expressing cells. Wortmannin, an inhibitor of PI 3-kinase, prevented MAP kinase activation and nuclear localization in middle-T-transformed cells. PI 3-kinase activity was also required for activation of the MAP kinase pathway in normal serum-stimulated cells, generalizing the concept that signaling through MAP kinases requires not only Ras-but also PI 3-kinase-mediated signals.

The role of yan in mediating the choice between cell division and differentiation

An allele of the yan locus was isolated as an enhancer of the Ellipse mutation of the Drosophila epidermal growth factor receptor (Egfr) gene. This yan allele is an embryonic lethal and also fails to complement the lethality of anterior open (aop) mutations. Phenotypic and complementation analysis revealed that aop is allelic to yan and genetically the lethal alleles act as null mutations for the yan gene. Analysis of the lethal alleles in the embryo and in mitotic clones showed that loss of yan function causes cells to overproliferate in the dorsal neuroectoderm of the embryo and in the developing eye disc. Our studies suggest that the role of yan is defined by the developmental context of the cells in which it functions. An important role of this gene is in allowing a cell to choose between cell division and differentiation. The relationship of the Egfr and Notch pathways to this developmental role of yan is discussed.

Phosphotyrosine residues in the nerve-growth-factor receptor (Trk-A). Their role in the activation of inositolphospholipid metabolism and protein kinase cascades in phaeochromocytoma (PC12) cells

PC12 cells, which lack platelet derived-growth-factor (PDGF) receptors, have been stably transfected with a chimaera consisting of the extracellular domain of the beta-PDGF receptor and the intracellular and transmembrane domains of the nerve-growth-factor receptor Trk-A (termed PT-R). Mutation of the Trk-A residue Tyr490 to phenylalanine prevents the association with Shc, while similar mutations at Tyr751 or Tyr785 are reported to prevent interaction of Trk-A with the p85 subunit of inositol phospholipid 3-kinase and phospholipase C-gamma 1, respectively. The strong and sustained activation of p42 and p44 mitogen-activated-protein kinases induced by PDGF-B/B in PC12/PT-R cells was unaffected by mutation of Tyr785 or Tyr751 to phenylalanine, but was smaller and transient after mutation of Tyr490, and almost abolished by the double mutation of Tyr490 and Tyr785. Mutation of Tyr490 reduced by 70% the PDGF-induced increase in inositol phospholipid 3-kinase activity immunoprecipitated from cell extracts with antiphosphotyrosine monoclonal antibodies and greatly suppressed the PDGF-induced increase in the intracellular products of inositol phospholipid 3-kinase, while mutation of Tyr751 or Tyr785 had no effect. Mutation of Tyr785 (but not mutation of Tyr490 or Tyr751) abolished PDGF-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate. Mutation of Tyr490, alone or in combination with mutation of Tyr751 and Tyr785, had no effect on the PDGF-induced activation of p70 S6 kinase (p70S6K). However, the activation of p70S6K by PDGF (or nerve growth factor), but not the activation of mitogen-activated-protein kinase, was prevented by two structurally unrelated inhibitors of inositol phospholipid 3-kinase, wortmannin or LY294002. Our results demonstrate the following: (1) the phosphorylation of Tyr490 plays a major role in the activation of inositol phospholipid 3-kinase and formation of 3-phosphorylated inositol lipids and confirm that the phosphorylation of Tyr 785 triggers the activation of phospholipase C-gamma 1 in vivo. (2) Tyr490 phosphorylation (but not inositol phospholipid 3-kinase activation) is also required for strong and sustained activation of mitogen-activated-protein kinase and neuronal differentiation, while the smaller and more transient activation of mitogen-activated-protein kinase, produced by the activation of phospholipase C-gamma 1 is insufficient to trigger the neuronal differentiation of PT-R cells. (3) Inositol phospholipid 3-kinase is required for the activation of p70S6K, but only a small increase in inositol phospholipid 3-kinase activity and the level of 3-phosphorylated inositol lipids is required for maximal p70S6K activation.

Early events in neurotrophin signalling via Trk and p75 receptors

Biological responses to neurotrophins appear to be mediated by multiple signalling pathways. These emanate from, and are regulated by, the contributions of both Trk and p75 receptors. Early events in Trk signalling are becoming more clearly defined and point to cooperate interaction of both Ras-dependent and Ras-independent pathways. Work over the past year has clarified the steps by which Trk receptor occupation leads to Ras activation and has highlighted the required roles of Ras and extracellular signal regulated kinases in certain neurotrophin responses, including neurite outgrowth. Pharmacologic and mutagenesis studies have additionally supported the importance of the phosphatidylinositol-3' kinase and SNT protein pathways in neurotrophin signalling. Although many findings point to clear involvement for p75 in neurotrophin signalling, the molecular mechanisms by which these occur are just beginning to be identified. Recent studies indicate that p75 dramatically influences Trk activity and ligand interactions, and may mediate signals through the ceramide second-messenger pathway.

Activation of p44 and p42 MAP kinases is not essential for the survival of rat sympathetic neurons

We have examined whether activation of MAP kinases [or extracellular signal-regulated kinases (ERKs)] is required for the survival of rat sympathetic neurons by comparing the actions of three survival factors whose survival-promoting actions can be blocked by neutralizing Fab fragments to p21 ras (Nobes and Tolkovsky, 1995, Eur. J. Neurosci., 7, 344-350), nerve growth factor (NGF), the cytokines ciliary neurotrophic factor (CNTF) and leukaemia inhibitory factor (LIF), and the cyclic AMP analogue 4-(8-chlorophenylthio)cAMP (CPTcAMP). NGF-induced survival was accompanied by an intense (15- to 30-fold) and steady (> 24 h) activation of p44 and p42 ERKs which waned rapidly (t1/2 approximately 30 min) upon NGF withdrawal. However, concentrations of NGF that induced a weak (4- to 5-fold) stimulation of the ERKs were not sufficient to maintain long-term survival. Moreover, prolonged and intense stimulation of the ERKs by NGF for up to 15.5 h was unable to confer long-term survival, since withdrawal of NGF after this time resulted in neuronal death that was kinetically indistinguishable from the death of neurons that had not been exposed to NGF. By contrast, CNTF and LIF continued to support survival for up to 3 days after eliciting only transient (< 30 min and 1 h respectively) activation of p44 and p42 ERKs, while CPTcAMP induced survival for several days without any measurable activation of the ERKs. Taken together, these data suggest that ERK activation per se is neither necessary nor sufficient for survival and that alternative pathways exist for effecting long-term survival of rat sympathetic neurons.

Role of the time factor in signaling specificity: application to mitogenic and metabolic signaling by the insulin and insulin-like growth factor-I receptor tyrosine kinases

The signal transduction pathways activated by hormones, growth factors, and cytokines show an extraordinary degree of cross-talk and redundancy. This review addresses the question of how the specificity conferred at the binding step is maintained through the signaling network despite the convergence of multiple signals on common efferent pathways such as mitogen-activated protein (MAP) kinase. The mechanism of receptor activation by ligand-induced dimerization provides a signaling device with both a switch and a timer. The role of the time factor, ie, of signaling kinetics, as a determinant of selectivity is discussed with emphasis on the receptor tyrosine kinases and cytokine receptors, and especially mitogenic versus metabolic signaling by insulin and insulin-like growth factor-I (IGF-I).

A proline-rich sequence unique to MEK1 and MEK2 is required for raf binding and regulates MEK function

Mammalian MEK1 and MEK2 contain a proline-rich (PR) sequence that is absent both from the yeast homologs Ste7 and Byr1 and from a recently cloned activator of the JNK/stress-activated protein kinases, SEK1/MKK4. Since this PR sequence occurs in MEKs that are regulated by Raf family enzymes but is missing from MEKs and SEKs activated independently of Raf, we sought to investigate the role of this sequence in MEK1 and MEK2 regulation and function. Deletion of the PR sequence from MEK1 blocked the ability of MEK1 to associate with members of the Raf family and markedly attenuated activation of the protein in vivo following growth factor stimulation. In addition, this sequence was necessary for efficient activation of MEK1 in vitro by B-Raf but dispensable for activation by a novel MEK1 activator which we have previously detected in fractionated fibroblast extracts. Furthermore, we found that a phosphorylation site within the PR sequence of MEK1 was required for sustained MEK1 activity in response to serum stimulation of quiescent fibroblasts. Consistent with this observation, we observed that MEK2, which lacks a phosphorylation site at the corresponding position, was activated only transiently following serum stimulation. Finally, we found that deletion of the PR sequence from a constitutively activated MEK1 mutant rendered the protein nontransforming in Rat1 fibroblasts. These observations indicate a critical role for the PR sequence in directing specific protein-protein interactions important for the activation, inactivation, and downstream functioning of the MEKs.

Cyclic adenosine monophosphate can convert epidermal growth factor into a differentiating factor in neuronal cells

The rat pheochromocytoma (PC12) cell line is a model for studying the mechanism of growth factor action. Both epidermal growth factor and nerve growth factor stimulate mitogen-activated protein (MAP) kinase in these cells. Recent data suggest that the transient activation of MAP kinase may trigger proliferation, whereas sustained activation triggers differentiation in these cells. We have tested this model by asking whether agents that stimulate MAP kinase without inducing differentiation can act additively to trigger differentiation. Neither forskolin nor epidermal growth factor can stimulate differentiation, yet both activate MAP kinase in these cells. Together, their actions on MAP kinase are synergistic. Cells treated with both agents differentiate, measured morphologically and by the induction of neural-specific genes. We propose that cellular responses to growth factor action are dependent not only on the activation of growth factor receptors by specific growth factors but on synchronous signals that may elevate MAP kinase levels within the same cells.

Increased expression and subcellular translocation of the mitogen activated protein kinase kinase and mitogen-activated protein kinase in Alzheimer's disease

The sequential activation of the mitogen-activated protein kinase kinase and its substrate, the mitogen-activated protein kinase is involved in a cascade of protein kinases which link a number of cell surface signals to intracellular changes in enzyme activity and gene expression. In vitro, mitogen-activated protein kinase is able to phosphorylate the microtubule-associated protein tau at Ser-Pro and Thr-Pro sites, thereby generating abnormally hyperphosphorylated tau species that are similar to paired helical filament-tau found in Alzheimer's disease. In the present study, we analysed the levels of immunoreactive mitogen-activated protein kinase kinase and mitogen-activated protein kinase in the temporal cortex (area 22) of patients with Alzheimer's disease by means of enzyme-linked immuno-sorbent assays and compared these changes with the content of abnormally phosphorylated paired helical filament-tau. The levels of immunochemically detected mitogen-activated protein kinase kinase and mitogen-activated protein kinase were both increased in Alzheimer's disease by between 35 and 40% compared with age-matched controls. Elevation of mitogen-activated protein kinase kinase was most pronounced during early stages of Alzheimer's disease and was inversely related to the tissue content of abnormally phosphorylated paired helical filament-tau. Pronounced immunoreactivity of mitogen-activated protein kinase kinase and mitogen-activated protein kinase was present in both tangle bearing neurons and unaffected neurons of the temporal cortex. Immunoreactive neurons were most often localized in the direct vicinity of neuritic plaques. In Alzheimer's disease, the subcellular distribution of mitogen-activated protein kinase kinase and mitogen-activated protein kinase showed a striking translocation from the cytoplasmic to the nuclear compartment. It is suggested that the activation of the mitogen-activated protein kinase cascade which appears to be an early feature of Alzheimer's disease might be critically involved in self-stimulating processes of neurodegeneration and aberrant repair under these conditions.

v-Crk modulation of growth factor-induced PC12 cell differentiation involves the Src homology 2 domain of v-Crk and sustained activation of the Ras/mitogen-activated protein kinase pathway

Nerve growth factor (NGF) and epidermal growth factor (EGF) elicit contrasting actions on PC12 pheochromocytoma cells; NGF causes neuronal differentiation, and EGF induces proliferation. However, ectopic expression of the Src homology 2 (SH2) and SH3-containing oncogenic adaptor protein v-Crk in PC12 cells results in EGF-inducible neuronal differentiation (Hempstead, B. L., Birge, R. B., Fajardo, J. E., Glassman, R., Mahadeo, D., Kraemer, R., and Hanafusa, H. (1994) Mol. Cell. Biol. 14, 1964-1971). Here we show that v-Crk complexes with both the tyrosine-phosphorylated EGF receptor and the Ras guanine nucleotide exchange factor SOS in PC12 cells and is involved in an pathway analogous to that of Grb2. Expression of v-Crk results in an enhanced and sustained activation of Ras and mitogen-activated protein (MAP) kinase following EGF or NGF stimulation, implying that v-Crk can couple divergent tyrosine kinase pathways to Ras. To investigate the causal relationship between EGF receptor binding, MAP kinase activation, and neurite outgrowth, we stably expressed two v-Crk SH2 point mutants, v-Crk(R273N) and v-Crk(H294R) in PC12 cells. Mutations within the SH2 domain of v-Crk block binding of v-Crk to the tyrosine phosphorylated EGF receptor, compromise v-Crk's ability to cause EGF-dependent neurite outgrowth, and act in a dominant negative manner for NGF-induced neurite outgrowth. However, the kinetics of MAP kinase activation in EGF- or NGF-treated v-Crk-(R273N)PC12 cells was comparable with that in v-CrkPC12 cells. These data are consistent with a model in which v-Crk regulates the strength of a tyrosine kinase signal leading to prolonged activation of Ras and MAP kinase. However, the experiments with the SH2 mutants suggest that sustained activation, by itself, may not be sufficient to switch the fate of v-CrkPC12 cells from proliferation toward differentiation.

Retrograde axonal transport of signal transduction proteins in rat sciatic nerve

Neurons require a mechanism to transmit stable signals over the large distance from the nerve growth cone or terminal to the cell body, in order that information from the target tissue can be relayed to the cell body where it is required. Nerve growth factor (NGF), a target-derived neurotrophic factor, is thought to signal over this distance by receptor mediated internalization of NGF, followed by retrograde axonal transport of the NGF-receptor complex. In this paper we show, by immunohistochemistry of rat sciatic nerve, accumulation of phosphotyrosine immunoreactivity only on the distal side of a nerve crush, suggesting axonal transport of tyrosine kinases and/or tyrosine phosphorylated proteins primarily in a retrograde direction. Furthermore, we also show retrograde axonal transport of phosphoinositide 3-kinase, ERK, MEK and MEK kinase, of which all but MEK kinase are known to be activated downstream of tyrosine receptor kinase activation. The retrograde transport of these proteins suggests that they may be involved in transmission of signals along the axon, relaying neurotrophic factor receptor activation at the nerve terminal to the nerve cell body.

The activation and nuclear translocation of extracellular signal-regulated kinases (ERK-1 and -2) appear not to be required for elongation of neurites in PC12D cells

The outgrowth of neurites was induced in PC12D cells, a subline of PC12 cells, that were treated not only with NGF but also with dbcAMP, staurosporine or bFGF. Simultaneous activation and rapid nuclear translocation of MAP kinases (ERK-1 and ERK-2) were observed in cells treated with NGF or bFGF. But staurosporine and dbcAMP induced no or only slight activation of the kinases. The nuclear translocation of the MAP kinases was not induced by the latter agents. These observations suggest a close relationship between the activation and the nuclear translocation of MAP kinases and, moreover, that stimulation and relocalization of MAP kinases might not be required for the outgrowth of neurites from PC12D cells. Staurosporine and dbcAMP may stimulate a down-stream step of the NGF pathway, or a parallel pathway(s) to the MAP kinase cascade in promoting neurite formation from PC12D cells. These agents mimic the effects of NGF in promoting neurite outgrowth in cultured sympathetic neurons, but not in conventional PC12 cells. Because of the similarity between PC12D cells and primed cells, it seems possible that activation and nuclear translocation of MAP kinases might be required for the transcription-dependent differentiation step but might not be necessary for the elongation of neurites at least in response to staurosporine or to dbcAMP.

Deletion of a conserved juxtamembrane sequence in Trk abolishes NGF-promoted neuritogenesis

Deletion of a conserved juxtamembrane sequence (KFG) in the Trk NGF receptor resulted in impaired neurite outgrowth, somatic hypertrophy, and induction of c-fos, c-jun, and TIS1 immediate-early genes. In contrast, these receptors retained the ability to mediate NGF-promoted survival and TIS8 and TIS11 immediate-early gene induction. The mutated receptor also mediated unimpaired autophosphorylation; SHC, PLC-gamma 1, and ERK tyrosine phosphorylation; and PI-3 kinase and ERK activation. However, SNT protein tyrosine phosphorylation, which wild-type receptors mediate via a ras-independent pathway, was undetectable. These findings indicate that the KFG sequence is indispensable for activating a ras-independent NGF signaling pathway involved in promoting neuronal differentiation and highlight potential roles of non-tyrosine-containing receptor domains in growth factor signal transduction.

Specificity of nerve growth factor signaling: differential patterns of early tyrosine phosphorylation events induced by NGF, EGF, and bFGF

The specificity of nerve growth factor (NGF) action was examined by comparing early tyrosine phosphorylation events induced by NGF, epidermal growth factor (EGF), and basic fibroblast growth factor (bFGF). In PC12 cells, administration of either the differentiation factor NGF or the mitogenic factor EGF led to tyrosine phosphorylation of multiple polypeptides in the 100-110 kDa size range associated with PI-3 kinase. However, NGF induced a more prolonged phosphorylation, relative to a transient EGF effect. In contrast, the differentiation factor bFGF failed to induce measurable tyrosine phosphorylation of PI-3 kinase-associated proteins. Similarly, NGF but not bFGF induced marked tyrosine phosphorylation of PLC gamma, another early signaling molecule, suggesting that multiple pathways exist for promoting differentiation, and/or that these signaling molecules are not essential for differentiation. TrkA signaling was also compared between PC12 cells and NIH-3T3 cells heterologously expressing trkA, where receptor activation promotes mitogenesis. In this comparison, significant differences were observed in the tyrosine phosphorylation pattern of PI-3 kinase-associated polypeptides, suggesting the existence of cell type-specific molecular interactions influencing trkA signaling. Mechanistically, NGF stimulation of PC12 cells resulted in a weak or possibly indirect association between trkA and PI-3 kinase. Furthermore, NGF did not appear to activate or substantially alter the overall level of PI-3 kinase activity, raising the possibility that ligand-induced phosphorylation may serve instead to relocalize constitutively active PI-3 kinase molecules within the cell. Taken together, data presented suggest that the temporal pattern of induced phosphorylation, the nature of induced associations with other phosphoproteins, and cell type-specific components may all contribute to the generation of NGF signaling specificity.

Stimulation of neurite outgrowth in PC12 cells by EGF and KCl depolarization: a Ca(2+)-independent phenomenon

MAP kinase activity is necessary for growth factor induction of neurite outgrowth in PC12 cells. Although NGF and EGF both stimulate MAP kinase activity, EGF does not stimulate neurite extension. We report that EGF, in combination with KCl, stimulates neurite outgrowth in PC12 cells. This phenomenon was independent of intracellular Ca2+ increases and not due to enhancement of MAP kinase activity over that seen with EGF alone. However, EGF plus KCl increased intracellular cAMP, and other cAMP elevating agents acted synergistically with EGF to promote neurite outgrowth. Stimulation of neurite outgrowth by cAMP and EGF was blocked by inhibitors of transcription suggesting that synergistic regulation of transcription by the cAMP and MAP kinase pathways may stimulate neurite growth.

XCL100, an inducible nuclear MAP kinase phosphatase from Xenopus laevis: its role in MAP kinase inactivation in differentiated cells and its expression during early development

We have cloned the Xenopus laevis homologue (XCL100) of the human CL100 (Thr/Tyr) MAP kinase phosphatase. Expression of the XCL100 mRNA and protein is inducible by serum stimulation and oxidative/heat stress in a X. laevis kidney cell line. In contrast, XCL100 is constitutively expressed in growing Xenopus oocytes. Recombinant XCL100 protein is able to dephosphorylate both tyrosine and threonine residues of activated p42 MAP kinase in vitro and both the Xenopus and human CL100 proteins were localised predominantly in the nucleus in transfected COS-1 cells. As nuclear translocation of activated MAP kinase is necessary for some of its essential functions in proliferation and cell differentiation our results indicate a role for CL100 in the regulation of these nuclear signalling events. In Xenopus kidney cells both heat shock and serum stimulation lead to transient activation of MAP kinase. However, in contrast to results previously reported from studies on mammalian fibroblasts the inactivation of MAP kinase in these epitheloid cells is rapid and is not dependent on synthesis of new protein. These results indicate that the induction of CL100 (or CL100-like enzymes) may not be required for MAP kinase inactivation in all cell types. Finally, during early embryogenesis, levels of XCL100 mRNA are greatly increased at the mid-blastula transition, suggesting that this enzyme may be involved in the regulation of MAP kinase activity during early development.

Involvement of MAP kinase in insulin signalling revealed by non-invasive imaging of luciferase gene expression in single living cells

BACKGROUND

Studies of the mechanisms by which signals are transmitted from receptor tyrosine kinases would be facilitated by a way of monitoring events at the single-cell level. We have explored how luciferase imaging can be used to examine the role of specific signalling pathways in insulin-stimulated gene expression. The analysis of luciferase expression in single cells has previously been hampered by the insensitivity of existing methodologies and the lack of a way of monitoring quantitatively, and independently, more than one promoter within the same cell. We have developed a technique for examining the dynamics of insulin-stimulated AP-1-dependent transcription in single living cells, and have explored the signalling pathway involved.

RESULTS

Luciferase and aequorin gene expression were examined in single living cells with a high-sensitivity photon-counting camera. The technique involved the comicroinjection of luciferase- and aequorin-based reporter plasmids directly into the cell nucleus, and the subsequent analysis of luminescence in the presence of luciferin and coelenterazine, respectively. The method is quantitative and allows insulin-stimulated gene expression to be monitored in real time. We found that insulin promoted a substantial increase in the expression of a luciferase gene under the control of the AP-1-binding site from the collagenase gene promoter. Aequorin expression, under the control of a cytomegalovirus promoter, was unaffected by insulin. The effect of insulin on luciferase expression was specifically blocked by overexpression of either the mitogen-activated protein (MAP) kinase phosphatase CL100, or the dominant-negative mutant MAP kinase kinase, MEKS217/221A.

CONCLUSIONS

Microinjection coupled with luciferase imaging allows hormone-regulated gene expression from relatively weak promoters to be monitored in single living cells. We have used this method to demonstrate that MAP kinase plays a central role in the ability of insulin to stimulate AP-1-dependent gene transcription.

Mitogen-activated protein kinase activation is insufficient for growth factor receptor-mediated PC12 cell differentiation

When expressed in PC12 cells, the platelet-derived growth factor beta receptor (beta PDGF-R) mediates cell differentiation. Mutational analysis of the beta PDGF-R indicated that persistent receptor stimulation of the Ras/Raf/mitogen-activated protein (MAP) kinase pathway alone was insufficient to sustain PC12 cell differentiation. PDGF receptor activation of signal pathways involving p60c-src or the persistent regulation of phospholipase C gamma was required for PC12 cell differentiation. beta PDGF-R regulation of phosphatidylinositol 3-kinase, the GTPase-activating protein of Ras, and the tyrosine phosphatase, Syp, was not required for PC12 cell differentiation. In contrast to overexpression of oncoproteins involved in regulating the MAP kinase pathway, growth factor receptor-mediated differentiation of PC12 cells requires the integration of other signals with the Ras/Raf/MAP kinase pathway.

Heterodimerization and functional interaction between EGF receptor family members: a new signaling paradigm with implications for breast cancer research

The EGF receptor (EGFR) and HER2 are members of a growth factor receptor family. Overexpression of either protein in advanced breast cancer correlates with poor prognosis. EGF stimulates growth by binding to EGFR, activating the receptor's intracellular tyrosine kinase. The initial consequence is phosphorylation of specific tyrosine-containing sequences in the receptor's carboxyl terminus. These phosphotyrosines serves as high affinity recognition sites for proteins that, in turn, transmit the growth signal inside the cell. Mechanistic studies suggest that EGF binds to a single EGFR, triggering dimerization with another like receptor molecule. This dimerization is thought to initiate the tyrosine kinase activation. The EGF receptor family was recently expanded with the sequencing of HER3 and HER4. Each of the four family members was postulated to regulate a unique growth or differentiation signaling repertoire when activated by a receptor-specific ligand. However, new data from numerous laboratories suggest that EGFR family members may play a complex and ultimately more flexible role in signaling by forming heterodimers between family members, e.g. EGFR:HER2 or HER4:HER2. These heterodimers may form even when only one member of the pair binds its ligand. This review summarizes current work on heterodimerization and attempts to predict the consequences for downstream signaling. In brief, when compared to ligand-dependent receptor homodimers comprised of two proteins with the same internalization sequence and phosphorylated tyrosine residues, heterodimers are likely to: i) expand substrate selection and downstream signaling pathway activation; ii) promote interaction between sets of substrates in the mixed receptor complexes that would not ordinarily be physically juxtaposed; iii) alter the duration of receptor signaling by changing rates of receptor internalization, ligand loss, kinase inactivation, recycling, etc.; and iv) alter rates of receptor and substrate dephosphorylation. In addition to understanding interactions of heterodimers with the internalization machinery, identification of receptor-specific substrates and binding proteins for each EGFR family member will be necessary to explicate the role of heterodimers in growth and differentiation.

Phosphorylation and localization of Kss1, a MAP kinase of the Saccharomyces cerevisiae pheromone response pathway

Kss1 protein kinase, and the homologous Fus3 kinase, are required for pheromone signal transduction in Saccharomyces cerevisiae. In MATa haploids exposed to alpha-factor, Kss1 was rapidly phosphorylated on both Thr183 and Tyr185, and both sites were required for Kss1 function in vivo. De novo protein synthesis was required for sustained pheromone-induced phosphorylation of Kss1. Catalytically inactive Kss1 mutants displayed alpha-factor-induced phosphorylation on both residues, even in kss1 delta cells; hence, autophosphorylation is not obligatory for these modifications. In kss1 delta fus3 delta double mutants, Kss1 phosphorylation was elevated even in the absence of pheromone; thus, cross-phosphorylation by Fus3 is not responsible for Kss1 activation. In contrast, pheromone-induced Kss1 phosphorylation was eliminated in mutants deficient in two other protein kinases, Ste11 and Ste7. A dominant hyperactive allele of STE11 caused a dramatic increase in the phosphorylation of Kss1, even in the absence of pheromone stimulation, but required Ste7 for this effect, suggesting an order of function: Ste11-->Ste7-->Kss1. When overproduced, Kss1 stimulated recovery from pheromone-imposed G1 arrest. Catalytic activity was essential for Kss1 function in signal transmission, but not for its recovery-promoting activity. Kss1 was found almost exclusively in the particulate material and its subcellular fractionation was unaffected by pheromone treatment. Indirect immunofluorescence demonstrated that Kss1 is concentrated in the nucleus and that its distribution is not altered detectably during signaling.

Shc binding to nerve growth factor receptor is mediated by the phosphotyrosine interaction domain

Shc is an adaptor protein that contains two phosphotyrosine-binding domains, a Src homology 2 (SH2) domain and the newly described phosphotyrosine interaction (PI) domain. Shc interacts with several tyrosine-phosphorylated proteins and is itself tyrosine-phosphorylated in cells stimulated with a variety of growth factors and cytokines. Upon phosphorylation, Shc binds to the Grb2.Sos complex leading to the activation of the Ras signaling pathway. Mutational analysis of the nerve growth factor (NGF) receptor (TrkA) suggested that the binding of Shc to the activated receptor is required for NGF-induced neuronal differentiation of PC12 cells. Here we report that the PI domain of Shc directly binds to tyrosine 490 on the autophosphorylated NGF receptor. The PI domain specifically recognizes an I/LXN-PXpY motif (where p indicates phosphorylation) as determined by phosphopeptide competition assay. In addition, the PI domain is able to efficiently compete for binding of full-length Shc proteins to the NGF receptor. In PC12 cells, the Shc SH2 domain interacts with an unidentified tyrosine-phosphorylated protein of 115 kDa but not with the activated NGF receptor. The ability of Shc to interact with different tyrosine-phosphorylated proteins via its PI and SH2 domains may allow Shc to play a unique role in tyrosine kinase signal transduction pathways.

A novel mitogen-activated protein kinase phosphatase. Structure, expression, and regulation

Mitogen-activated protein (MAP) kinase lies at the convergence of various extracellular ligand-mediated signaling pathways. It is activated by the dual-specificity kinase, MAP kinase kinase or MEK. MAP kinase inactivation is mediated by dephosphorylation via specific MAP kinase phosphatases (MKPs). One MKP (MKP-1 (also known as 3CH134, Erp, or CL100)) has been reported to be expressed in a wide range of tissues and cells. We report the identification of a second widely expressed MKP, termed MKP-2, isolated from PC12 cells. MKP-2 showed significant homology with MKP-1 (58.8% at the amino acid level) and, like MKP-1, displayed vanadate-sensitive phosphatase activity against MAP kinase in vitro. Overexpression of MKP-2 in vivo inhibited MAP kinase-dependent gene transcription in PC12 cells. MKP-2 differed from MKP-1 in its tissue distribution and in its extent of induction by growth factors and agents that induce cellular stress, suggesting that these MKPs may have distinct physiological functions.

Inhibition of MAP kinase kinase blocks the differentiation of PC-12 cells induced by nerve growth factor

The mitogen-activated protein kinase (MAP kinase) pathway is thought to play an important role in the actions of neurotrophins. A small molecule inhibitor of the upstream kinase activator of MAP kinase, MAP kinase kinase (MEK) was examined for its effect on the cellular action of nerve growth factor (NGF) in PC-12 pheochromocytoma cells. PD98059 selectively blocks the activity of MEK, inhibiting both the phosphorylation and activation of MAP kinases in vitro. Pretreatment of PC-12 cells with the compound completely blocked the 4-fold increase in MAP kinase activity produced by NGF. Half-maximal inhibition was observed at 2 microM PD98059, with maximal effects at 10-100 microM. The tyrosine phosphorylation of immunoprecipitated MAP kinase was also completely blocked by the compound. In contrast, the compound was without effect on NGF-dependent tyrosine phosphorylation of the pp140trk receptor or its substrate Shc and did not block NGF-dependent activation of phosphatidylinositol 3'-kinase. However, PD98059 completely blocked NGF-induced neurite formation in these cells without altering cell viability. These data indicate that the MAP kinase pathway is absolutely required for NGF-induced neuronal differentiation in PC-12 cells.

Cell cycle reentry of mammalian fibroblasts is accompanied by the sustained activation of p44mapk and p42mapk isoforms in the G1 phase and their inactivation at the G1/S transition

Mitogen-activated protein (MAP) kinases are serine/threonine kinases that are rapidly activated in response to mitogenic stimuli. Here we examined the enzymatic activity and phosphorylation state of the individual p44mapk and p42mapk isoforms during early G1 and late G1 phase of the mammalian cell cycle. Release of fibroblast cells from early G1 block was accompanied by a rapid rise in the myelin basic protein (MBP) kinase activity of p44mapk and p42mapk, which declined slowly over several hours to reach negligible values as cells enter S phase. When cells were released from late G1 block, the activity of p44mapk and p42mapk increased transiently, and then rapidly declined to baseline values during G1 to S phase transition. Cells released at the G1/S boundary in a medium lacking growth factors entered S phase in the complete absence of MAP kinase activity. Unlike MAP kinases, the histone H1 kinase activity of p33cdk2 was elevated in late G1 arrested cells and continued to increase during S phase entry. The enzymatic activation of p44mapk and p42mapk in both early G1 and late G1 phase was accompanied by an increase in the phosphothreonine and phosphotyrosine content of the proteins. These findings suggest that the sustained activation of MAP kinases during G1 progression and their inactivation at the G1/S transition are two regulatory processes involved in the mitogenic response to growth factors.

TrkA tyrosine residues involved in NGF-induced neurite outgrowth of PC12 cells

The proto-oncogene product gp140prototrk (TrkA) is the receptor tyrosine kinase that mediates nerve growth factor-induced neuronal survival and differentiation. In receptor tyrosine kinases, specific intracellular tyrosine residues become phosphorylated after ligand binding and the phosphorylated tyrosines induce the cascade of signal transduction. Here we have identified intracellular tyrosine residues of TrkA involved in nerve growth factor-induced neurite outgrowth of PC12 cells, using site-directed mutagenesis and a PC12 cell line expressing very low levels of endogenous TrkA (PC12nnr5 cells). We analysed eight conserved intracellular tyrosine residues of TrkA while the three putative autophosphorylation sites conferring tyrosine kinase activity were left intact. Five tyrosine residues, Y499, Y643, Y704, Y760 and Y794, in rat TrkA were involved in nerve growth factor-induced neurite outgrowth. None of these tyrosines mediated the full activity of wild-type TrkA, and a pair of these tyrosines, Y760 and Y794, promoted neurite outgrowth in an additive manner. These data indicate that no single tyrosine is sufficient to induce complete neurite outgrowth but the five tyrosine residues Y499, Y643, Y704, Y760 and Y794 cooperate to exhibit the full activity of wild-type TrkA.

K252a-potentiation of EGF-induced neurite outgrowth from PC12 cells is not mimicked or blocked by other protein kinase activators or inhibitors

Epidermal growth factor (EGF) has recently been shown to cause certain strains of PC12 cells to extend short neurites. This EGF-induced differentiation of PC12 was found to be potentiated by the protein kinase inhibitor, K252a, in that PC12 cells treated with both EGF and K252a extended long branched neurites similar to those induced by nerve growth factor (NGF). As reported here no other protein kinase inhibitor or activator mimicked or blocked the effect of K252a on EGF-induced PC12 differentiation. Cyclic adenosine 3',5'-monophosphate (cAMP) also potentiated EGF-induced neurite outgrowth from PC12 cells, but the mechanism of this potentiation was different from that of K252a. Cells that had been exposed to EGF and then stripped of their neurons extended neurites again when retreated with EGF in the absence of RNA synthesis or when treated with NGF in the absence of RNA synthesis. Thus EGF can prime PC12 cells for either EGF or for NGF, a finding that further suggests that EGF and NGF use similar signaling pathways to induced neuronal differentiation of PC12.

Staurosporine causes epidermal growth factor to induce differentiation in PC12 cells via receptor up-regulation

Although they all utilize tyrosine kinase receptors and activate signaling pathways characterized by a similar set of phosphoproteins, epidermal growth factor (EGF) promotes only cell division while fibroblast growth factor (FGF) and nerve growth factor (NGF) can induce division followed by differentiation in PC12 cells. EGF, in contrast to NGF and FGF, cannot maintain the sustained phosphorylation and activation of mitogen-activated protein (MAP) kinase kinase and MAP kinases, which may account for the difference in phenotypic response. The pretreatment of PC12 cells with staurosporine, a protein kinase inhibitor, causes a substantial increase in both receptor and MAP kinase phosphorylation that results in a differentiative response (neurite proliferation). However, neurites begin to disappear after 3 days, despite the continual presence of EGF, and are largely gone after 5 days, which is not the case with NGF and FGF. Thus, the effect of staurosporine is not permanent. Northern and Western blots indicate that the staurosporine response mainly results from a substantial up-regulation in EGF receptor synthesis, thus providing a much stronger cell surface signal and supporting the view that quantitative rather than qualitative differences distinguish the EGF versus NGF/FGF signaling pathways in these cells.

Transforming growth factor beta activation of p44mapk in proliferating cultures of epithelial cells

Transforming growth factor-beta (TGF-beta) is a potent growth inhibitor of a variety of epithelial cell types. The primary signaling mechanism involved in mediating this and other cellular effects of TGF-beta is still unknown. We report here that both TGF-beta 1 and TGF-beta 2 resulted in a rapid activation of mitogen-activated protein kinase (MAPK) p44mapk, occurring within 5-10 min of growth factor addition. This effect occurred in exponentially proliferating cultures of intestinal epithelial (IEC) 4-1 cells under conditions in which DNA synthesis was inhibited by 95% to 98%. Furthermore, TGF-beta 2 induced a sustained activation of p44mapk under these conditions, lasting for at least 90 min after initial growth factor treatment. Another TGF-beta-sensitive epithelial cell line (CCL 64) displayed a similar rapid increase in p44mapk activity when treated with TGF-beta 1. In contrast, in IEC 4-6 cells that are resistant to TGF-beta effects on growth and DNA synthesis, TGF-beta 2 treatment did not result in an activation of p44mapk. In contrast to the results in proliferating cultures, treatment of quiescent cultures of IEC 4-1 cells with TGF-beta 2 resulted in no significant change in either DNA synthesis or p44mapk activity within 15 min of TGF-beta addition. In contrast, addition of the growth-stimulatory combination of factors (epidermal growth factor + insulin + transferrin = EIT) to quiescent and proliferating IEC 4-1 cells stimulated DNA synthesis and resulted in a sustained activation of p44mapk. Together, our results suggest an association between activation of p44mapk and both TGF-beta-mediated growth inhibition and EIT-mediated growth stimulation. This suggests that the specificity for the cellular effects of growth factors may not occur at the level of MAPK activation per se, but rather at downstream events that include phosphorylation of distinct transcriptional complexes and activation of a select assortment of genes. With regard to TGF-beta specifically, we have proposed a model to explain how activation of p44mapk may be associated with a growth-inhibitory response.

v-Abl-mediated apoptotic suppression is associated with SHC phosphorylation without concomitant mitogen-activated protein kinase activation

A temperature-sensitive mutant of the v-Abl protein has previously been shown to exhibit tyrosine protein kinase activity in Interleukin 3 (IL-3)-dependent IC.DP cells grown at the permissive temperature (32 degrees C) but not at the restrictive temperature (39 degrees C). These IC.DP cells are dependent on IL-3 for suppression of apoptosis at 39 degrees C, but at 32 degrees C cells will survive without added growth factor. Both IL-3 and v-Abl stimulated the tyrosine phosphorylation of SHC and GTPase-activating protein. However, while IL-3 stimulated similar levels of tyrosine phosphorylation in p46shc and p52shc, v-Abl preferentially phosphorylated p52shc, an event that occurred within 1 h of temperature switch. v-Abl also differentially associated with p46shc in a temperature-independent manner. In contrast, only IL-3 stimulated detectable increases in both myelin basic protein kinase and mitogen-activated protein (MAP) kinase kinase in in vitro assays, although in more specific MAP kinase activity assays a very slight increase in the activity of this enzyme was observed after 6 h at the permissive temperature. Time course studies suggest that phosphorylation and association of SHC with v-Abl is insufficient to lead to significant activation of MAP kinase and that activation of the MAP kinase kinase/MAP kinase pathway is not required for apoptotic suppression.